The present invention is directed to an article of manufacture and method of use to reduce particulate matter and carbon monoxide emissions from a cooking fire. Particularly, the present invention is concerned with a solid-fuel-fired cookstove, used for cooking, heating, and lighting, that incorporates a non-platinum group metal (PGM), oxidation catalyst and a passive damper. Here “PGM” refers to ruthenium, rhodium, palladium, osmium, iridium, or platinum. The oxidation catalyst lowers the oxidation temperature to promote complete combustion to carbon dioxide, while the passive damper controls the air-to-fuel ratio to improve thermal efficiency, encourage catalyst light off, and promote complete combustion to carbon dioxide.
A significant percentage of the world's population, estimated at 2.5 billion people, regularly cooks with biomass fuels such as wood or charcoal. In many countries, the demand for firewood outpaces forest regrowth, leading to deforestation. Additionally, emissions from biomass cookstoves contribute to global climate change, increase indoor air pollution, and are harmful to human health. Exposure to high indoor air pollutant levels from cooking with biomass fuels is responsible for an estimated 1.6 million deaths annually and about 3% of the global burden of disease. As such, it is important to improve fuel efficiency in order to reduce both deforestation and harmful combustion-derived emissions.
The most basic cookstove is the so-called three-stone fire that consists of a cooking vessel balanced upon three stones of similar height, with the fire located at the center of the stones. Numerous stoves exist that improve fuel efficiency and combustion efficiency compared to the three-stone fire. Stove designs vary widely in size, shape, design, and material. At minimum, most stoves incorporate a combustion chamber, an internal exhaust conduit, and a pot support or drip pan that the cooking vessel rests upon. Optionally, a chimney can be attached to some stoves to improve draft and direct stove exhaust outside of the home. Here “chimney” is defined as the duct that carries exhaust out of the home and is located downstream of the location where heat is extracted for the purpose of cooking and “exhaust” is defined as the mixture of unreacted air, pollutants, and combustion products that are produced during the combustion of solid fuel.
Prior art stoves include small stoves with a single burner and large stoves with multiple burners. Large stoves contain the fire within a combustion chamber and smoke is exhausted through a long chimney that is not exposed to flames and is not integral to the stove, but rather installed separately. In contrast, the small stoves either lack a chimney or the chimney is a few inches long and is regularly exposed to flames. The incorporation of a catalyst within large cookstoves is known in the prior art. However, a key requirement of large cookstoves in the prior art is that the catalyst not be exposed to flames. Similarly, while small catalytic camping stoves using gaseous fuels and PGM catalysts are known in the art, a small, solid-fuel-fired cookstove incorporating a catalyst exposed to flames is not known in the prior art.
Most known large cookstoves incorporating a catalyst typically use a PGM oxidation catalyst deposited on a ceramic substrate. PGMs are typically excellent oxidation catalysts and are the most common catalysts used to treat automobile exhaust. However, PGM catalysts add significant cost. In addition, large cookstove prior art teaches that the catalyst must be separated from any flames to ensure catalyst longevity, either by locating the catalyst deep in a long chimney or by locating the catalyst in a secondary combustion chamber free from flames and extreme temperatures. Here the “secondary combustion chamber” is the conduit for exhaust gases between the combustion chamber and the location where heat is extracted for the purpose of cooking. This prior art therefore actively teaches away from the present invention in which the catalyst is exposed to flames.
In the present invention, the oxidation catalyst is positioned directly above the fire in a chimneyless cookstove. Placing a catalyst in this position leads to certain challenges that must be overcome, such as the lack of natural draft caused by the lack of a chimney and potential catalyst damage caused by flame impingement. The prior art has not incorporated an oxidation catalyst in proximity to a cookstove's flame. We recognized that using typical, commercial, ceramic-supported, PGM catalysts would not be suitable for use in smaller stoves where the catalyst is subjected to direct flame impingement. Rather, an inexpensive oxidation catalyst supported on a refractory substrate is required for a small, chimneyless catalytic cookstove. Here “refractory” refers to material properties that include resistance to decomposition by heat, physical wear, and chemical attack. Additionally, the short nature of the stove requires that the catalyst not hinder the limited natural draft. Therefore, the catalyst must be supported on a substrate with a minimal pressure drop.
Oxidation catalysts lower the activation energy needed to oxidize a given reactant. In the case of complete combustion, a fuel is oxidized to form relatively harmless carbon dioxide and generate thermal energy. In a cookstove, this thermal energy is used to cook food. However, combustion is typically an incomplete reaction, leading to the formation of carbon monoxide, partially oxidized organics, and particulate matter, e.g. soot. Both carbon monoxide and particulate matter are harmful to humans when inhaled. An oxidation catalyst can promote the conversion of carbon monoxide and particulate matter to form carbon dioxide. Therefore introducing an oxidation catalyst to a cookstove will reduce the concentration of carbon monoxide and particulate matter and directly mitigate health problems associated with cooking over an open flame.
Rather than a PGM catalyst, a low-cost, chimney-free cookstove intended for developing communities requires a significantly less expensive oxidation catalyst. Potassium is a relatively inexpensive and readily abundant element that can also act as an oxidation catalyst, but potassium needs to be stabilized by other elements to remain active for long periods of time. There are several potassium compounds that are effective oxidation catalysts. We found potassium titanate to be particularly active and stable, but other low-cost compounds are also suitable for this purpose. These other low-cost compounds can include crystalline and amorphous metal oxides with a single metallic element, such as manganese oxide (MnO, Mn2O3, Mn3O4), or with multiple metallic elements such as potassium-silicon-calcium glass, potassium strontium titanate (SrTiO3, Sr0.8K0.2TiO3), potassium cobalt oxides (K0.25CoO2), potassium copper oxides (KCuO), potassium cobalt cerium oxides, and the like. Additionally, these low-cost compounds can be doped with metallic elements like copper, cobalt, cerium, and the like to further improve catalytic activity. An example of such a low-cost compound is potassium titanate, which has the chemical formula K2O.nTiO2 (n=2, 4, 6, 8). More specifically, K2Ti2O5 is effective and reduces particulate matter ignition temperature by 100° C. Potassium titanate has been described in the prior art as a catalyst used to oxidize soot produced by diesel engines and as a NOx storage and reduction (NSR) catalyst. However, K2Ti2O5 has not been identified previously as a catalyst for reducing emissions from solid-fuel combustion.
Compared to particulate matter, carbon monoxide is more difficult to oxidize. The bond strength of the carbon-oxygen triple bond (1072 kJ/mol) found in carbon monoxide is significantly greater than the bond strength of carbon-carbon double bonds (602 kJ/mol) found in particulate matter. Surprisingly, we have shown that K2Ti2O5 is also active for carbon monoxide oxidation. The carbon monoxide oxidation activity of K2Ti2O5 is enhanced further when doped with either copper or cobalt. When doped with either of these transition metals, K2Ti2O5 lowers the carbon monoxide oxidation temperature by 200° C. compared to the undoped K2Ti2O5 catalyst.
Potassium titanate is typically produced as a powder, and therefore must be supported on a substrate to be integrated within a cookstove. The catalyst substrate must form a strong bond with potassium and be refractory. The substrate can be metal or ceramic. A metal substrate is generally preferred because ceramics are brittle and further embrittled by high temperatures, leading to poor durability. The metal can be a pure element or an alloy, but must have a high melting point. In one embodiment of the present invention, an iron-chromium-aluminum alloy is used for the substrate because the alloy is durable and demonstrates good adhesion with potassium titanate. However, other metallic and ceramic substrates would also be suitable for this purpose.
The substrate can take on many different shapes or designs. The primary requirement is that the pressure drop caused by the substrate is small enough to allow proper airflow through the cookstove. The pressure drop through the substrate is governed primarily by the substrate void fraction, thickness, and channel diameter. Here “void fraction” is defined as the ratio, expressed as a percentage, of voids to total volume. Here “thickness” is defined as the length of the substrate in the direction of exhaust flow. Here “channel diameter” is defined as the average hydraulic diameter of void spaces in between substrate structure. A substrate void fraction greater than 65%, a substrate thickness less than 1.5 inches, and a channel diameter in the range of 0.1 inches to 0.3 inches is suitable for purposes of the present invention. Monolithic catalyst substrates with a honeycomb pattern or a corrugated spiral pattern are near ideal. Here “honeycomb pattern” is defined as having parallel channels or holes, which are separated by many thin walls, where the channels can be square, hexagonal, round, or other shapes. Additional substrates can include beads of any shape, a mesh or system of meshes, wires and rods, raschig rings, or similar types of packing. In one embodiment of the present invention, a metal monolithic substrate with a corrugated spiral pattern has been found to be suitable. However, other substrate form factors having a high void fraction may also be suitable.
An additional element of the present invention is a passively controlled damper that self-regulates the air flow based on fuel size. Here “passive control” is defined as control that does not use any sensors or actuators and does not consume any power. Previously disclosed dampers are either manually controlled by the stove operator, or are automatically controlled by a temperature sensor and a powered actuator. Manually controlled dampers are used to switch between a fire starting position and a fire burning position. Manual dampers with a range of positions can also be used to optimize airflow for different fire temperatures or burn rates. However, these active dampers require regular manual adjustment by the stove operator or some type of automated switching mechanism. The prior art describes stoves with automatic damper controls incorporating thermoswitches and powered actuators to adjust the damper position based upon the fire temperature.
In one embodiment of the present invention, the passive damper comprises a flow obstruction attached to a hinge mechanism and suspended at the top of the fuel inlet duct. The passive damper can also take the form of a curtain or shroud-like barrier composed of flexible, vertically hanging, fire resistant strands. In operation, the fuel paced in the inlet duct impinges upon the flow obstruction, and causes the flow obstruction to swing open. The larger the size of the fuel, the more the flow obstruction is forced open. If no passive damper is present, using small pieces of fuel wood in the cookstove will result in an excess of air flowing through the stove due to the large open cross-sectional area for the buoyancy-driven combustion air draft. When a passive damper is used with small pieces of fuel, the passive damper swings down, partially blocking the flow, and preventing excess air from traveling through the stove. This allows the stove to operate more efficiently, as thermal energy can go towards heating food rather than heating excess air. Also, the damper promotes higher exhaust temperatures, and encourages light-off of the catalyst (i.e., achieving the minimum temperature required for catalytic surface oxidation reactions). When large amounts of fuel are used, the passive damper swings up, or otherwise moves out of the way, to allow large fuel wood to enter the cookstove. As the size of the fuel pieces is reduced through burning, the damper closes to meet the smaller fuel pieces and thus self-adjusts. Because the fuel blocks a portion of the inlet that corresponds to the size of the fuel pieces, airflow into the stove will remain at desired levels regardless of the size or amount of fuel wood used in the stove.
An object of this invention is to incorporate a non-PGM catalyst within a cookstove to reduce the emission of carbon monoxide and particulate matter.
A further object of this invention is to incorporate a passive damper that adjusts airflow through a cookstove based on the size of solid fuel used.
Yet another object of this invention is to integrate with buoyancy driven or forced-convection solid-fuel cookstoves, whether biomass or fossil-fuel fired.
Yet another object of this invention is that the catalyst is a potassium titanate that can be further doped with either copper or cobalt.
Yet another object of this invention is that the catalyst is adhered to a solid, porous, refractory support structure, or substrate, suspended above the cookstove fire.
Yet another object of this invention is that the passive damper controls airflow by physically blocking a portion of the fuel inlet cross section.
Yet another object of this invention is that the passive damper can rotate or otherwise move to clear the inlet and allow the entry of larger pieces of fuel.
The present invention improves on the known variations of solid-fuel cookstoves by incorporating a non-PGM catalyst and a passive damper to improve combustion efficiency and reduce emission of carbon monoxide and particulate matter.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings and non-limiting examples herein.